Sharpies bleed badly on fabrics. Ask me how I know. I would get a set of fabric paint markers. Whatever you use, don't let the point sit on the fabric because it will soak in. Some people use the crayons, but I do like the more vivid colors. Let the finished piece air dry over night or a couple of night, then press from the back side with a hot iron. But a piece of muslin or something so that the colors don't transfer to your ironing board.
Can't wait to see what they look like all 'prettied up'. Be sure to post pictures.